1. Field of the Invention
The present invention relates to a pad layout structure of a semiconductor chip, and more particularly, to a pad layout structure of a semiconductor chip capable of preventing lead-broken problems when packaging the semiconductor chip in a tape carrier package.
2. Description of the Related Art
A tape carrier package (hereinafter, referred to as TCP) is obtained by applying a wireless bonding technique among techniques for packaging a highly integrated semiconductor chip. In general, the semiconductor chip is mounted on a TCP tape in which leads are formed as wires.
Since the TCP does not use separate bonding means such as gold wires unlike a conventional package that requires a wire bonding process, it is possible to obtain an inexpensive and compact package.
However, it is difficult to apply the TCP to a case where there are a large number of pads, and a pad pitch is small. For example, in case of a source driver chip for driving a liquid crystal panel, a large number of input and output pads for supplying power and transmitting signals are arranged in an area around an internal circuit of the source driver chip. However, the area for arranging the pads is limited with respect to the source driver chip. In order to arrange the input and output pads in the limited area, the pad pitch needs to be designed to a minimum within an allowable range for a procedure of manufacturing the semiconductor chip. Since the pad pitch in the semiconductor chip is designed to the minimum, a width of leads formed on the TCP tape is also small so as to package the semiconductor chip in a TCP.
As the width of leads decrease, a strength of leads also decreases. In case of a semiconductor chip with a low aspect ratio, although the width of the leads is decreased when packaging the semiconductor in a TCP, considerable problems do not occur. However, in case of a semiconductor chip with a high aspect ratio, for example, a ratio equal to or greater than 10:1, such as a source driver chip with an extremely high aspect ratio, lead-broken problems may occur due to even weak impact exerted on both edges of the semiconductor chip.
In order to prevent the lead-broken problems, the longitudinal length of the semiconductor chip has to be increased. Accordingly, it is difficult to miniaturize the semiconductor chip, since the longitudinal length of the semiconductor chip is increased without adding circuits.
FIG. 1 illustrates an example of a conventional pad layout structure of a semiconductor chip.
Referring to FIG. 1, an internal circuit 110 is disposed at the center of a semiconductor chip 100. Output pads 102 are arranged at upper, lower, left and right sides 120a, 120b, 120c, and 120d of the internal circuit 110. An input pad unit 101 is disposed at the center of the upper side 120a. As is not shown, the input pad unit 101 includes a plurality of input pads. The output pads 102 with a predetermined width W1 are arranged at a predetermined spacing S1. When the width of the pads and the spacing between neighboring pads are constant, a pad pitch represented by a sum of the width and the spacing is also constant.
As described above, in a case where the semiconductor chip 100 of FIG. 1 is a source driver chip, the semiconductor chip 100 generally has a long shape with a high aspect ratio. As shown in FIG. 1, in a case where the output pads are arranged at a predetermined pad pitch P1, lead-broken problems may occur at both edges 130 of the semiconductor chip 100 as described later.
FIG. 2 illustrates difference in moment between semiconductor chips due to difference in length between semiconductor chips.
Referring to FIG. 2, in case of (a), a length L1 from the center 210a to the edge 220a of a semiconductor chip 200a is relatively short. In case of (b), a length L2 from the center 210b to the edge 220b of a semiconductor chip 200b is relatively long. A moment of a semiconductor chip is proportional to a value obtained by multiplying a force F by the length L1 or L2 from the center 210a or 210b to the edge 220a or 220b of the semiconductor chip 200a or 200b. Accordingly, even when the same force F is applied to the output pads 102, the moment generated at the edge 220b of the semiconductor chip 200b is greater than the moment generated at the edge 220a of the semiconductor chip 200a. 
Accordingly, even when the same force is applied to the output pads 102 due to a bonding tool when packaging a semiconductor chip 100 in a TCP, the moment generated at pads arranged at the edges 130 of the semiconductor chip 100 which are relatively far from the center of the semiconductor chip 100 is greater than the moment generated at pads arranged to be relatively near the center of the semiconductor chip 100. Since the moment is transferred to the leads formed on the TCP tape, the force applied to the leads arranged at the edges of the semiconductor chip 100 exceeds a yield strength of the leads. Thus, the leads may be broken.